This invention relates to thermoforming open end articles such as jars at orientation temperature.
It is broadly known to form high strength hollow articles such as jars and the like from polymer sheet which has been heated to orientation temperature as is shown by Dockery, U.S. Pat. No. 3,461,503. It is also known in the conventional thermoforming art to achieve greater uniformity of wall thickness by sucking the plastic up against a concave bottom plug face or billowing it out from the plug face as shown, for instance, by Rowe, Jr., U.S. Pat. No. 2,990,581.
However, it has been found that the technology which has grown up around the ordinary thermoforming of molten sheet is not necessarily applicable to thermoforming of sheet at orientation temperature. For instance, molten polyethylenes, when contacted with a plug assist, tend to exhibit no slippage past the periphery of the plug assist. This is why it is necessary in order to achieve thinning of the area under the plug assist to utilize techniques such as are shown in Rowe, Jr. Since the molten plastic under the plug assist is not going to stretch due to slippage, it is necessary to stretch it in some other way, such as by drawing it up into a deep concave surface as is shown in FIG. 3 of Rowe, Jr. However, this identical polymer, when heated to orientation temperature, has a tendency to slip past the periphery of the plug and further is not sufficiently pliable to be easily billowed back and forth; thus, techniques for achieving uniformity which work well in thermoforming molten sheet are not useful in thermoforming at orientation temperature.